Drink bottled water

A common recommendation for good reason. Bottled water is safe in most cases, but... Look out for tampering. Sometimes vendors in poor countries will refill the bottle with boiled or even tap water. Which means you'll head for dysentery central.

Watch out for altitude sickness

Mexico City is high enough (at 2,250 meters / 6,500 feet above sea level) that many visitors will suffer from mild altitude sickness. Symptoms include headaches, loss of appetite and the inability to sleep well. If you plan to visit places at high altitude, remember that it will take a day or two for your body to adjust. Take a lot of rest breaks, drink water, take a mild painkiller for headaches and restrict time spent in direct sunlight. It is also easier to suffer from heat exhaustion at high altitude.